Workaround for bug in pip

Author: jpadilla

tox.ini

@@ -15,7 +15,7 @@ deps =
        django16: Django==1.6.3  # Should track minimum supported
        django17: Django==1.7.2  # Should track maximum supported
        django18: Django==1.8  # Should track maximum supported
-       djangomaster: https://github.com/django/django/zipball/master
+       djangomaster: https://github.com/django/django/archive/master.tar.gz
        -rrequirements/requirements-testing.txt
        -rrequirements/requirements-optionals.txt